That's a very much "it depends" answer.
On the older 3750, software is provided in two feature sets, IPBase and IP Services. Which to chose, depends on your license. This software also comes in multiple versions, newer versions having more features. Generally, we often chose the oldest, still supported (with maintenance) train that provides the features we need. Why oldest? It's been out the longest, and so, often it's the most stable. The 12.2(55) version recommended by Balaji is a very stable version, although support on that train might have stopped.
On the newer 3650, (I believe?) software again comes in two versions, IPBase and Universal, the latter is needed for any features beyond provided by IPBase, although it works for that feature set too (just smaller in size). For that, same recommendation as before for which to chose.
BTW, Cisco often tags a version as recommended.
PS:
Just took a look at software.
The 3750G's IOS isn't listed, but for the 3750V2 (which may [?] work on a 3750G), Cisco recommends 12.2(55)SE12. (Again, this was recommended by Balaji.)
For the 3650, Cisco recommends 16.9.5 (as also recommended by Balaji). (Also I was mistaken, this IOS only has a universal version.)
